This invention relates to security glazing units and to alarm systems particularly suitable for detecting attempts to break or tamper with glazing installations. The invention may be used as a burglar alarm or as a security system in penal institutions or the like.
Security alarm systems of the prior art conventionally include a frangible conductor, such as a foil tape or thin wire, typically in a serpentine or circumscribing path, affixed to the window glazing and through which an electric current is passed. When the window is broken, the conductor also breaks, thereby opening the circuit and triggering an alarm. A typical system employing thin wires may be seen in U.S. Pat. No. 1,223,583, and a typical system employing foil tape may be seen in U.S. Pat. No. 3,409,886. Such arrangements possess certain disadvantages however. In order to keep power consumption low, the conductor can be made relatively large in width or diameter so as to reduce its resistance, but the use of such obstrusive conductors can be unsightly, particularly if the center portion of a window is to be protected. Alternatively, the path of the conductor may be shortened in order to lower the resistance of the circuit, but large areas of a window may then be left unprotected, making it possible to partially break or cut through the window without disturbing the conductor. An attempt to avoid this dilemma is shown in U.S. Pat. No. 3,441,925, where a transparent conductive coating is applied to the entire window area. This arrangement still requires a relatively large break in the window in order to register a definite change in the resistance. Small holes and cracks in the glass do not produce discrete, predictable changes in the resistance, and when laminated glazing is used in such an arrangement (as is normally required to protect the conductive coating) major gaps in the circuit are not likely to be opened without an extraordinarily forceful and wanton attack. This is especially true in the case of laminated units designed to resist penetration.
Another disadvantage of most prior art security systems is that once one of the conductors has been broken, the system cannot be reset until the damage is repaired, which can involve a considerable lapse of time, particularly if a window must be reglazed. In the meanwhile no security is provided for the window where the break occurred, even though the damage may have been caused accidentally and may be slight. A system is disclosed in U.S. Pat. No. 1,766,500 that includes two independently actuatable circuits, so that if one wire is broken, a second wire still provides security. But that arrangement requires extra electrical components and provides only minor reset capability, limited to the case of only one wire being broken. If two wires happen to be broken instead of just one, the system will be put out of commission just the same. The circuit of another prior art system, shown in U.S. Pat. No. 2,423,649, includes two resistances in parallel, but there is no suggestion therein to extend a plurality of frangible conductors across an area of entry so as to provide backup protection after an attempted breach of security. Another related circuit is shown in U.S. Pat. No. 3,138,792 in a system that does not include frangible wires.
Accordingly, there is a need for a security system that can reliably detect security breaches in the early stages and then be reset to quickly re-establish security. At the same time, it is desirable that visual aesthetics not be sacrificed in security glazing installations.